1. Field of the Invention
The invention relates to a method for forming lightly doped diffusion regions, and more particularly to a method for forming lightly doped diffusion regions for dense mask read-only memory devices.
2. Description of the Prior Art
Memory capacities in digital systems are usually expressed in terms of bits, since a separate storage device or circuit is used to store each bit of data. Each storage element is referred to as a cell. Memory capacities are also sometimes stated in terms of bytes or words. Each byte typically represents an alphanumeric character. Every bit, byte or word is stored in a particular location, identified by a unique numeric address, and only a single bit, byte, or word is stored or retrieved during each cycle of memory operation.
Memory-storage capability is expressed in units of kilobits and megabits (or kilobytes and megabytes). Since memory addressing is based on binary codes, capacities that are integral powers of 2 are typically used. As a result, a memory device with a 1-kbit capacity can actually store 1024 bits, and a 64-kbit device can store 65536 bits. In digital computers, the number of memory bits is usually 100 to 1000 times greater than the number of logic gates, which implies that the memory cost per bit must be kept very low. In addition, it is desirable for the memory devices to be as small as possible, to operate at a high speed, to have a small power consumption, and to operate reliably.
When the memory devices are considered for specific functions, such as high speed or reliability, the addition of lightly doped diffusion region structure is necessary. Typically, there are two methods for forming lightly doped diffusion regions in a semiconductor structure. One is formation of spacer oxide, such as spacers on a gate structure. The other is implemented by salicide process. However, those methods are sot complicated that may raise the manufacture costs of the memory devices.
Accordingly, it is an object of the present invention to provide a method for forming lightly doped diffusion regions. The lightly doped diffusion regions are formed without depending on formation of spacer oxide or salicide process.
It is another further object of the present invention to provide a method for forming lightly doped diffusion regions in mask read-only memory devices. In the present invention, a photoresist layer is formed on a former formed photoresist layer and then etched back for forming a spacer-shaped mask at the sidewalls of the former formed photoresist layer.
In one embodiment, a method for forming lightly doped diffusion regions in a semiconductor read-only memory comprises providing a silicon substrate of a first type conductivity. A first photoresist layer, having a lightly doped diffusion region pattern, is formed on the silicon substrate. Next, dopants of the first type conductivity are implanted into the silicon substrate for forming a lightly doped diffusion region in the silicon substrate. A second photoresist layer is formed on and at, the surface and sidewalls of the first photoresist layer, and on the surface of the silicon substrate. Then the second photoresist layer is etched back and dopants of a second type conductivity are implanted into the silicon structure for forming a source/drain region underlying the lightly doped diffusion region in the silicon substrate.